One of the more important recent developments in the petroleum refining arts is the establishment in the prior art of new processes to produce high octane gasolines blended with lower aliphatic alkyl ethers as octane boosters and supplementary fuels. C.sub.5 -C.sub.7 methyl alkyl ethers, especially methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) have been found particularly useful for enhancing gasoline octane. Therefore, improvements to the processes related to the production of these and similar ethers in the course of gasoline production are matters of high importance and substantial challenge to research workers in those arts.
It is known that alkyl tert-alkyl ethers can be prepared by reacting a primary alcohol with an olefin having a double bond on a tertiary carbon atom, thus methanol reacts with isobutylene and isopentenes (2 methyl 1-butene or 2 methyl 2-butene) to form respectively methyl tert-butyl ether (MTBE) and methyl tert-amyl ether (TAME). The reaction is selective for tertiary olefins so that it constitutes a valid process for their removal from olefinic streams in which they are contained together with linear unreactive olefins. The reaction has an equilibrium which is favorable to the synthesis of the ether as the reaction temperature is lowered, in accordance with the reactions negative enthalpy.
It is known that the reaction is catalyzed by Lewis acids (aluminum trichloride, boron trifluoride), mineral acids (sulfuric acid) and organic acids (alkyl and aryl sulfonic acids, ion exchange resins). Particularly suitable for the task are ion exchange resins in their acid form and it is known that the best results are obtained by means of macroreticular resins of the type "Amberlyst 15". By means of such last named catalysts it is possible to reach thermodynamic equilibrium within industrially acceptable contact times in the temperature range of 50.degree.-60.degree. C.
U.S. Pat. No. 4,262,145 to Selwitz et al. discloses the catalytic reaction of a branched olefin such as isobutylene, 2-methylpentene-2, 2-methylbutene-2, and 2,3-dimethyloctene-2 with a lower alkanol such as methanol to form a mixed ether such as methyl tert-butyl ether. The catalyst disclosed is silicotungstic acid.
A process is also known for manufacturing ethers from linear mono-olefins, thereby augmenting the supply of highoctane blending stock for gasoline. The lower molecular weight ethers, such as methyl isopropyl ether, are in the gasoline boiling range and are known to have a high blending octane number. U.S. Pat. No. 4,714,787 to Bell et al., incorporated herein by reference in its entirety, provides a catalytic process for selectively reacting one or more linear mono-olefins with a primary or secondary lower molecular weight alcohol to form the corresponding ether. The active acidic catalyst component for the process is selected from the group consisting of sulfonated ion-exchange resins and crystalline silicates having a pore size greater than 5 angstrom units. Of the crystalline silicates, those preferred include crystalline zeolites having a silica to alumina mol ratio greater than about 12. In a particularly preferred embodiment, methanol and propylene are reacted to selectively form methyl isopropyl ether (MIPE).
A preferred feedstock for the manufacture of MTBE and TAME in petroleum refinery operations is the light hydrocarbon stream from FCC operations. These streams are rich in C.sub.4 +tertiary olefins such as isobutylene. However, they also contain significant amounts of linear olefins plus linear and branched paraffins. The linear olefins, particularly propylene and n-butene, are not etherified in the prior art MTBE processes. Conventionally, these linear unreacted olefins are carried through the process and separated downstream. In this regard they represent a burden on the volumetric effectiveness of the etherification process, providing no assistance to the production of ether-rich high octane gasoline.
Processes for the conversion of olefins as well as oxygenates to gasoline are well-known in the prior art. The processes for the conversion of methanol to olefins and olefins to gasoline are but representative of a series of analogous processes based upon the catalytic capabilities of zeolites. It is known that zeolites, such as ZSM-5, can convert methanol to hydrocarbons of higher average molecular weight. Depending on various conditions of space velocity, temperature and pressure methanol, and lower oxygenates in general, can be converted in the presence of zeolite type catalyst to olefins which may then oligomerize to provide gasoline or distillate or may be converted further to produce aromatics.
The feasibility and adaptability of the basic chemistry of zeolite oxygenates conversion to produce useful conversion processes has been the subject of much inventive research activity. Recent developments in zeolite catalyst and hydrocarbon conversion processes have created interest in using oxygenates and olefinic and paraffinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, aromatics, etc. In addition to the basic work derived from ZSM-5 type zeolite catalyst, a number of discoveries have contributed to the development of a new industrial process. This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components into higher hydrocarbons over crystalline zeolites having controlled acidity. Reaction conditions of moderate severity favor the conversion of olefins to predominantly gasoline boiling range products with little paraffins conversion. Milder reaction temperatures and high operating pressures can produce distillate range fuels as well from lower olefins. Garwood et al. have also contributed improved processing techniques in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above identified disclosures are incorporated herein by reference.
Accordingly, it is an object of the present invention to provide a process for the enhanced conversion of olefinic components of hydrocarbon streams containing mixed branched and linear olefins to ether-rich high octane gasoline.
It is another object of the present invention to provide an integrated process for the sequential conversion of tertiary olefins and linear olefins to lower alkyl ethers and the conversion of unreacted olefins to gasoline.
Yet another object of the instant invention is to provide an integrated process for converting light hydrocarbons comprising mixed olefins plus paraffins to high octane ethers and gasoline.